1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device, and more particularly, to an LCD device including an optical component for changing the polarization state of light emitted from the LCD device.
2. Discussion of the Related Art
LCD devices have become the ubiquitous human/device interface. This is particularly true in electronic devices such as mobile phones, game devices, electronic dictionaries, video cameras, digital cameras, and MP3 players. LCD devices are also used as computer monitors and in notebook computers. LCD TVs using LCD devices are very popular and are available in large sizes. Thus, humans spend many hours each day viewing text and video images on LCD devices.
Referring to FIGS. 1A and 1B, an LCD includes two glass substrates having transparent electrodes 102, 104. The glass substrates are spaced by glass beads or plastic beads with a diameter of several microns, and the edges thereof are sealed with an adhesive. Twisted nematic liquid crystals 130 may be injected and twisted by 90° between the upper and lower substrates 104, 102. Two light polarizers 120, 110 having mutually perpendicular optical axes are respectively attached to the outer surfaces of the upper and lower substrates 104, 102. The optical axis (in parallel with the paper plane) of the upper polarizer 120 is parallel to the orientation of the liquid crystals at the upper substrate 104, and the optical axis of the lower polarizer 110 (perpendicular to the paper plane) is parallel to the orientation of the liquid crystals at the lower substrate 102. Natural light, e.g., from a light source not shown, is incident into a liquid crystal screen in a direction from the upper substrate 104 to the lower substrate 102. When no electrical field is applied (FIG. 1A), the light passes through the first polarizer 120 to become light polarized in parallel with the orientation of the liquid crystals at the upper substrate 104. The polarized light is then rotated 90° by the liquid crystal layer to be parallel to the optical axis of the polarizer 110 at the lower substrate 102 so as to pass through the polarizer. This is referred to as the bright state of the display.
As shown in FIG. 1B, when an electrical field is applied, the liquid crystal molecules 130 are rotated to align themselves in a direction of the electrical field, and the originally twisted orientation turns into a vertically parallel orientation. The polarized light does not interact with the vertically aligned liquid crystals, and the polarization plane will not be rotated when the polarized light emitted from the upper polarizer 120 passes through the liquid crystal layer. When the polarized light reaches the polarizer 110 at the output end, the polarization axis of the polarizer is perpendicular to the polarization direction of the output light. Light is blocked from being emitted from the polarizer 110 in what is called the dark state of the LCD device. If the electrical field is not strong enough to reorient the liquid crystals completely, the liquid crystal molecules will be oriented with an angle of inclination and the light rotation effect will occur to some degree. A portion of the light will pass through to exhibit an intermediate grayscale.
The above describes the operation principle of an LCD device. It is known from the operation principle of the LCD device that the output light will be linearly polarized. Also, in addition to the twisted nematic liquid crystals, the liquid crystal material will have some distortion characteristics such as extension and bending, and the different distortion characteristics correspond to different display modes of the LCD device. However, the basic principles of these display modes are similar to that of the twisted nematic liquid crystals, i.e., changing polarization directions of the light to display light patterns through different orientations of the liquid crystal molecules. Therefore, the final output light is a linearly polarized light.
Unlike linearly polarized light, natural light has a uniform vector distribution in all directions. The stimulation of the uniformly distributed natural light on photoreceptor cells in human eyes is isotropic, and human eyes are more accustomed to natural light. Circularly polarized light is also a kind of polarized light, however, the vector directions of circularly polarized light are rotating rapidly, and so circularly polarized light also exhibits characteristics of isotropism like natural light. Compared with linearly polarized light, human eyes feel more comfortable receiving and viewing circularly polarized light.
Over the past decade, with the rapid development of information technology and the great increase in human-computer interaction, there have been increasing complaints of eye fatigue and other eye irritations. There also have been reports relevant to this issue establishing that the eye fatigue phenomenon is especially prevalent among people who spend long hours using computers or watching LCD TVs.
LCD display technology has been identified as a cause of eye fatigue in part because of the relatively high average brightness of the LCD devices such as TVs, and the blurriness of rapidly moving images on LCD TVs. Also, since the light emitted from LCD TVs is linearly polarized, it contributes to eye fatigue and irritation. Therefore, it is of great significance to change the polarization state of the output light of conventional LCD devices from being linearly polarized to being circularly polarized similar to natural light to alleviate eye fatigue and irritation of the viewers.